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Thursday, May 23, 2013

Flowery pancake animal

At low tide, well out towards the bottom of the intertidal zone, we often find sand dollars. They're usually hard and whitish; the bare, empty skeletons of the living animal. Rarely, we see a live one. These are black, and have spines all over, top, bottom, and edges. When I look closely, I can see the spines moving.

We found one of these last week, and I brought it home to add to the aquarium, where it settled in happily, burrowing slowly under the sand.

I fished it out two days later, to see if it had survived the transfer. It had; it was trying to walk out of my hand, on its spine "feet".

Top side. One edge is broken.

Under the light, it wasn't really black. And where the edge is regrowing, the spines had a pink tinge to them. When I had a photo, I checked the coloring; the main colour is muted down so it looks brown or black, but when I saturated the photo, the spines were all red.

Detail, central "flower".

I looked at the spines under my little microscope. They're beautiful, transparent as glass, but with dotted lines running the length of each spine. There are two types; long, tapered pillars, and between them, short, thin hairs with a knob on top. And all of them, all the time, are swaying in all directions.

They are far too small for my camera, but I found a photo on the web that is more or less what I saw.

Back in the tank, the spines went to work, sliding the sand dollar over and under the sand until it disappeared from view.

For a flat plate, moving on microscopic fur, the dollar manages to cover a lot of territory. Here's a time-lapse video of one in California, from mrjustin5 on YouTube.

This was about 25 min(ute)s of footage condensed into 1 min 30 s.

A mini-Roomba! All it needs is a mini-kitten.

The sand dollar can live up to 10 years. This one is about 3 inches across, half the size of the largest I've seen, so it's probably still young. I hope it does well in the tank, away from its hungriest predators, starfish and gulls. Hermit crabs will eat them, too, so I'll watch for that and remove it if necessary.

8 comments:

Love the sand dollars! Related to sea stars & sea urchins, these cool little creatures are built similarly, although that's not readily apparent until one studies them closely, as you have. The spines are each mounted to the body frame with a sort of ball & socket joint and can rotate 360 degrees around. The tiny "threads" between the spines are actually their tube feet, which act as suction cups to move the animal around and help move food to its mouth in the center of the underside. Mostly, they feed on detritus, often situating themselves on edge in the sand to catch the food that floats in with tidal action. Hopefully, you have enough of that kind of food in your tank to support one.

Although I rarely take the time to comment, I enjoy your blog very much!

Ladybug, thank you! The page where I found the microphotograph was very confusing in its explanation of the two types of spines. It seems that they both have various functions, including walking, but only the smaller ones have suction tips. At least in the Australian species the author describes, the tube feet can also elongate to reach beyond the larger spines.

That's a great video, probably took a long time to get that much motion. I remember going to Ensenada and seeing hundreds (probably more like multi-thousands) on the sand flats. You could barely walk without crushing them under your feet. - Margy

A few weeks ago, we visited Boundary Bay at low tide (< 1m) and saw many live sand dollars in the shallow tide pools. It was my first time seeing them in their natural habitat, resting nearly vertically, half immersed in the sand. I'll post a writeup when I can get around to it.

Tim, Looking forward to it! We don't often get that far out any more, because Laurie has difficulty walking any distance because of an accident. (Falling off a ladder onto his back. Luckily, nothing was broken.)